The present invention relates to a polyvinyl alcohol multifilament yarn (abbreviated as PVA yarn hereinafter) and a process for producing the same. More particularly, it relates to a PVA yarn which has high tensile strength together with good hot water resistance, and to a process for producing the same.
The most required characteristics of the yarn using in the industrial field application such as tire cords, ropes, stitching threads, seat belts and fishing nets, is high tensile strength. In order to get a high tensile strength PVA yarn which is usable in the industrial field, U.S. Pat. Nos. 4,603,083 and 4,698,194 have been already proposed. As these PVA yarns proposed therein have a degree of polymerization of at least 1500, a tensile strength of at least 12 g/d and an initial modulus of at least 280 g/d, they are sufficient in mechanical properties, and usable in the industrial field application.
However, these PVA yarns have insufficient hot water resistance so that they melt down when they are used at high temperature in presence of water. So they are apparently better in tensile strength than conventional PVA yarns, but not entirely sufficient in the field of tire cord application which is required a stable reinforcing effect for a long period.
In order to improve hot water resistance of a PVA yarn, there has been already proposed some methods. For example, according to Japanese Patent Publication No. 46-11457, it is tried to improve the hot water resistance of a PVA yarn by mixing polyvinyl alcohol with boron compound to make the crosslinked structures between hydroxyl groups of polyvinyl alcohol and boron. However, the hot water resistance of the PVA yarn according to the above method was insufficient in the tire cord application which is used in very severe conditions.